The present invention relates to a chemical mechanical polishing (to be referred to as CMP hereinafter) technique for planarizing the surface of, e.g., a semiconductor wafer and, more particularly, to a supplying apparatus for supplying a liquid mixture containing a polishing agent and chemicals to a polishing pad, a supplying method thereof, and a semiconductor polishing apparatus and method employing this liquid mixture supplying apparatus or method.
In recent years, along with a trend for a higher packing density and finer micropatterning of semiconductor devices, various types of micropatterning techniques have been studied and developed. Among these techniques, the CMP technique is indispensable in planarization of an insulating interlayer film, formation of a plug, formation of a buried metal wire, isolation of buried elements, and the like.
FIG. 4 shows the schematic arrangement of a CMP apparatus used in the manufacture of a semiconductor device.
More particularly, a polishing table 101 can rotate in the direction of an arrow in FIG. 4 about a shaft 102 as the center. A polishing pad 103 is adhered to the upper surface of the polishing table 101. A wafer carrier 104 for holding a semiconductor wafer is arranged above the polishing table 101 at a position shifted from the center of the polishing table 101. The wafer carrier 104 can rotate in the direction of an arrow in FIG. 4 about a shaft 105 as the center.
The wafer carrier 104 is mounted on the shaft 105. The wafer carrier 104 has a diameter smaller than the radius of the polishing table 101 and, e.g., a vacuum chuck. A semiconductor wafer 106 is held by the lower surface of the wafer carrier 104 with vacuum chucking. The polishing surface of the semiconductor wafer 106 held by the wafer carrier 104 is urged with a predetermined pressure so as to come into contact with the surface of the polishing pad 103.
A tank 107 is arranged above the polishing table 101. The tank 107 stores a polishing liquid 109 adjusted in advance. The polishing liquid 109 in the tank 107 is supplied to the surface of the polishing pad 103 through a pump (P) 108 and a pipe 110. The distal end of the pipe 110 is arranged near the center of the polishing pad 103 and upstream a position where the semiconductor wafer 106 comes into contact with the polishing pad 103.
To polish a wafer with the CMP apparatus having the above arrangement, first, the polishing table 101 is constantly rotated in one direction at a predetermined rotation speed about the shaft 102 as the center. In this state, through the pipe 110, the polishing liquid 109 stored in the storage tank 107 is supplied upstream a position on the polishing table 101 where the semiconductor wafer 106 comes into contact with the polishing pad 103.
Thereafter, the wafer carrier 104 is moved downward while being constantly rotated in one direction at a predetermined rotation speed about the shaft 105 as the center, to cause the polishing surface of the semiconductor wafer 106 to come into contact with the polishing surface of the polishing pad 103 with a predetermined pressure. The polishing surface of the semiconductor wafer 106 is brought into slidable contact with the polishing surface of the polishing pad 103 through the polishing liquid 109, so that it is gradually polished and planarized.
In the CMP apparatus having the above arrangement, however, since the polishing liquid 109 adjusted in advance is stored in the tank 107 and is supplied from the tank 107 through the pipe 110, the following problems arise.
In the conventional CMP apparatus, a polishing liquid 109, which is obtained by suspending, e.g., polishing particles, in a solution, e.g., water, added with a chemical substance, is generally used. Therefore, as time passes, the polishing particles aggregate or are precipitated in the tank 107 to clog the pipe 110, or reaction or decomposition of the chemical substance proceeds exceedingly to change the properties of the polishing liquid 109.
For example, if the polishing liquid 109 is a mixture of SiO.sub.2 having an average primary particle diameter (initial particle diameter) of about 0.05 .mu.m and hydrogen peroxide, the average secondary particle diameter of initial SiO.sub.2 stored in the tank 17 is about 0.3 .mu.m. When a cumulative storage time of 0.5 hour has elapsed, the average secondary particle diameter of SiO.sub.2 has grown to about 0.7 .mu.m, and about 10% of polishing particles are precipitated. Accordingly, hydrogen peroxide having a comparatively high decomposition speed is wasted for about 10% a day.
Therefore, the pipe 110 must be prevented from clogging with the aggregated or precipitated polishing particles or the polishing liquid 109 must be prevented from being changed in properties by the reaction or decomposition of the chemical substance, and the polishing liquid 109 having a stable particle diameter and a stable concentration must always be supplied. For this purpose, the polishing liquid 109 may be exchanged often so that it will not be stored over a long period of time, or the polishing liquid 109 may be adjusted and stored in the tank 107 in batches of small amounts. However, this operation is cumbersome and is not practical. In addition, when the polishing liquid is exchanged often to maintain the stability of the polishing liquid 109, every time the operation is to be performed, the CMP apparatus must be stopped. This greatly degrades the operating efficiency of the CMP apparatus to interfere with an increase in manufacturing efficiency.
In order to solve these problems, a solution in which polishing particles are suspended and a solution added with a chemical substance may be separately supplied onto the polishing pad, and may be mixed on the polishing pad to generate the polishing liquid. In this case, however, the solution in which the polishing particles are suspended and the solution added with the chemical substance cannot be mixed sufficiently. Accordingly, it is difficult to supply a stable polishing liquid, and the uniformity in polishing is impaired due to the variations in concentration of the polishing liquid.
FIG. 5 schematically shows another CMP apparatus disclosed in U.S. Pat. No. 5,407,526.
In this CMP apparatus, a tank 202 stores a solution 201 in which polishing particles are suspended, and a tank 204 stores a solution 203 added with a chemical substance. The solution 201 pumped from the tank 202 with a pump 205 and supplied to a pipe 206 and the solution 203 pumped from the tank 204 with a pump 207 and supplied through a pipe 208 are mixed by a mixing unit 209. A polishing liquid 210 generated by the mixing unit 209 is supplied onto a polishing pad 103.
In the CMP apparatus having the above arrangement, the polishing liquid 210 is generated by the mixing unit 209, and thereafter it is supplied to the surface of the polishing pad 103. Therefore, a cumbersome operation for exchanging the polishing liquid is not required, thus improving operating efficiency. However, since the polishing liquid 210 is generated by the mixing unit 209, the polishing liquid 210 may stay in the mixing unit 209 undesirably. More specifically, in order to sufficiently mix the solutions 201 and 203, the diameter of the discharge port of the mixing unit 209 is smaller than the inner diameter of the mixing unit 209. Accordingly, the polishing liquid 210 stays in the mixing unit 209 to cause aggregation or precipitation of the polishing particles. As a result, the mixing unit 209 may clog, or a change in properties of the polishing liquid 210 due to the reaction or decomposition of the chemical substance cannot be prevented completely.